(1) Field of the Invention
The present invention relates to a transmission device and transmission method that use a multilevel modulation system to transmit and receive data via a wire or wireless channel.
(2) Description of the Related Art
One type of encoder used in a transmission device that uses a two-level modulation system is the 8B/10B encoder disclosed in Patent Literature 1. For an eight-bit sequence that is input, the 8B/10B encoder limits the maximum number of identical consecutive values to a predetermined value or less (hereinafter, “guarantee the run length”) and outputs a 10-bit encoded bit sequence for which the difference in the number of occurrences of two modulation conditions in a specified period of time is limited to a predetermined value or less (hereinafter, “guarantee the DC balance”). This 8B/10B encoder is used in communication systems starting with Ethernet™.
However, when the above 8B/10B encoding system is used with multilevel modulation that has more than two modulation conditions, e.g. when using four-level modulation, then the run length and the DC balance are not guaranteed. For example, an encoded bit sequence “0101010101”, whose run length and DC balance are guaranteed in the case of two-level modulation, would in the case of four-level modulation be interpreted as containing the value “01” five times continuously, since one modulation condition is allocated every two bits. Therefore, neither the run length nor the DC balance would be guaranteed.
When the run length is not guaranteed, clock recovery in the reception circuit is not possible since a modulation change does not occur, and thus the problem occurs that demodulation in the reception device cannot be performed properly. Also, when DC balance is not guaranteed, the problem occurs that the identification level in the reception circuit changes due to DC offset, making correct judgment of data impossible, and therefore demodulation in the reception device cannot be performed properly. Another problem occurs in that the scale of the circuit grows larger, since a circuit to compensate for the DC offset becomes necessary in the reception device.
It is an object of the present invention to provide a transmission device, transmission method, and circuit that, in a communication system that uses multilevel modulation with 2 ^n levels (n being an integer greater than or equal to two), limit the run length to a predetermined value or less and guarantee DC balance.    Patent Literature 1    Japanese Patent Application Publication No. 59-10056